1. Field of the Invention
This invention relates generally to detection circuits that can be used with optical touch panels and more particularly to a circuit which compensates for the noise signals created by ambient light variations and by electromagnetic interference.
2. Description of the Prior Art
Prior arrangements which have been proposed for touch panels are disclosed in U.S. Pat. No. 3,764,813, granted Oct. 9, 1973, U.S. Pat. No. 3,775,560, granted Nov. 27, 1973, and U.S. Pat. No. 4,198,623, granted Apr. 15, 1980. In systems of the type disclosed in the above patents, the interruption of closely spaced beams of light is employed to locate the coordinates of a point which is being pointed to on the invisible opto-matrix; and semiconductor switching circuits are often employed to sequentially turn on successive pairs of opposing photosource and photodetectors, to thereby scan across the face of the display, both horizontally and vertically. A common output circuit is connected from all of the photodetectors sampling switches which are sequentially energized to sense whether or not there is anything, such as a finger, blocking one or more of the light beams. The photodetector output circuit normally extends substantially along two edges of a circuit board, and therefore, can be subject to electromagnetic interference, from voltages, currents and radiation typically within the proximity of the touch panel housing. In addition, the photodetectors are affected by variations in the ambient lighting conditions where the unit is located, and these may also adversely affect the operation of the unit.
Heretofore, one approach disclosed in U.S. Pat. No. 4,243,879, granted on Jan. 6, 1981, used a digital sample and hold technique to mitigate the effects of variations in the level of ambient light by sampling ambient light level as seen by the phototransistor just before the photodetector is energized. While some protection was provided for variation in ambient light it does not compensate for electromagnetic interference.
Accordingly, it is a principal object of the present invention to provide an improved photodetection circuit which is more reliable and more readily compensates for both electromagnetic interference and ambient light variations than those which have been employed heretofore.